Bashikaogong-Shimierbulake granitic complex, north Altun, NW China: Geochemistry and zircon SHRIMP ages

Abstract

The Bashikaogong-Shimierbulake granitoid complex is about 30 km long and 2-6 km wide, with an area of 140 km 2 , located at the north margin of the Bashikaogong Basin in the north Altun terrain. It intruded into schist, metapelite and metatuff of Precambrian ages. This granitoid complex consists of darkish quartz diorite, grey granite, pink granite and pegmatite. Geochemically, the quartz diorite has I-type granite affinity and belongs to Calc-alkaline sereies, and the other granites have S-type affinity and to high-K calc-alkaline series. Zircon SHRIMP U-Pb dating shows that the quartz diorite has a bigger age than those of other granites, which is 481.6±5.6 Ma for quartz diorite, 437.0±3.0 Ma-433.1±3.4 Ma for grey granite and 443±11 Ma-434.6±1.6 Ma for pink granite, respectively. Combined with regional geology, we think that the quartz diorite formed in tectonic environment related to oceanic crust subduction and the granites in post-collision.The basement of Milanhe-Jinyanshan in the North Altun terrain consists of a suit of Precambrian metamorphic rocks including gneiss, schist and highpressure metapelite [1] . A number of mafic and ultramafic rocks include basalt and pillow lavas, as well as siliceous rocks (tuff and granite) [2] , which is a subduction-collision complex belt characterized by ophiolite, abyssal and bathyal clastic rocks, carbonate rocks and metamorphic rocks. Nearly W-E striking fault-bounded high-pressure metapelite, 1-5 km wide and about 40 km long, occurs as thin-beded or lens-shaped layers in eastern Beiketan, central part of the subduction-collision belt. It has been suggested to result from subduction of a continental crust to 80 km deep and later exhumation to near surface [3,4] . The basic rocks are mainly transitional MORB-type basalts with minor OIB-type basalts formed at about 508.13±41Ma-524.14±44Ma (Sm-Nd isochron age) [1,2,5] . The granites intruded into the gneissic wall rocks either as relatively large plutons or small-scale veins. Wallrock-derived enclaves are common in the proximity of the contact between granitic plutons and the wallrocks. Kuoshibulake granitie, one of the largest in the subduction-collision complex belt has a TIMS zircon U-Pb age of 443 Ma [6] and was believed to be formed in either an island or a continental arc setting [7] . Petrology, geochemistry and geochronology

Opening of an early Paleozoic limited oceanic basin in the northern Altyn area: Constraints from plagiogranites in the Hongliugou-Lapeiquan ophiolitic mélange

“…The age of 448.6 ± 3.3 Ma for the MilanHongliugou pillow lavas reported by Xiu et al [12] was similar to the post-collision S-type granites [12][13][14][15][16], which is contradictory to the tectonic evolution of the belt. Nevertheless, Yang et al [11] reported the age 479.4 ± 8.5 Ma for a gabbro at Hongliugou, which probably represented a younger oceanic crust formation event.…”

“…For example, there are the zircon U-Pb age of 481.6 ± 5.6 Ma for the island arc quartz diorites in the southern belt, 481.5 ± 5.3 Ma of the granodiorites in the northern belt, and the ages of 437, 433, 411 and 434 Ma for the post-collision S-type granites age in the southern belt [13][14][15][16]. In summary, the previous studies indicated that the HLOMB had experienced a complex tectonic evolution.…”

Petrology and geochronology of HP–UHP rocks from the South Altyn Tagh, northwestern China

“…Phengite and paragonite separated from blueschist and eclogite yield consistent Ar-Ar plateau/ isochron ages of 491-513 Ma . SHRIMP U-Pb dating of zircons from gneissic granodiorite yielded a metamorphic age of 482 ± 5 Ma (Wu et al, 2006). These ages and index fossils suggest that this complex mainly formed during the Early Paleozoic.…”

The early Paleozoic sedimentary–tectonic evolution of the circum-Mangar areas, Tarim block, NW China: Constraints from integrated detrital records

“…From the N-S seismic section crossing well TZ29, Ordovician-Silurian strata show a wedge-shaped foreset structure with an obvious inclination towards north (Zhao et al, 2010), indicating that the clastic materials are derived from the areas south of the Tarim basin. Massive ages of 450-440 Ma from collision-related magmatic rocks and high-ultrahigh pressure metamorphic rocks have been reported during recent years from the North Altyn Tagh and the North Qilian (Zhang et al, , 2007Chen et al, 2003;Gehrels et al, 2003;Song and Niu, 2004;Wu et al, 2005Wu et al, , 2006Liu et al, 2009), which were well consistent with the global 'Caledonian movement' and stand for the end of the Proto-Tethys tectonic realm, implying that the North Altyn Tagh and the North Qilian did yield vast detrital sources for the Mangar depression. Conclusively, the extinction of the Mangar depression was affected combinedly by the opening of the South Tianshan back-arc basin to the north and the subductioncollision of the North Altyn Tagh and the North Qilian.…”